Heretofore, there has been known a light emitting device composed by combining a solid-state light emitting element and a wavelength converter including a phosphor with each other. As such a light emitting device as above, for example, there has been known a white LED light source as described in PTL 1 or a laser lighting device and a laser projector, which are as described in PTL 2 and PTL 3. Note that a large number of light emitting devices, each having a light source that applies a laser beam, have a structure in which phosphor particles are adhered to a flat surface of a substrate having the flat surface, and in general, perform high light density excitation of the phosphor (for example, refer to PTL 6).
In a light emitting device that uses blue light as an excitation source of the phosphor, a garnet-based phosphor activated with Ce3+ is frequently used. A typical example of this garnet-based phosphor is a YAG-based phosphor, and is represented by a general formula Y3Al2(AlO4):Ce3+. Note that this YAG-based phosphor becomes a phosphor that radiates green light by replacing a part of Al in a crystal lattice thereof by Ga or replacing a part of Y in the crystal lattice by Lu. Moreover, it has also been known that this YAG-based phosphor becomes a phosphor that radiates yellow light by replacing a part of Y in the crystal lattice by Gd (for example, refer to PTLs 1 and 5).
A Ce3+-activated amount of the YAG-based phosphor to be used for the light emitting device using the solid-state light emitting element as an excitation source of the phosphor is usually 2 to 3 atomic % with respect to a total number of rare earth ions in the crystal lattice (for example, refer to PTL 1).
Moreover, a particle size (when defined by a median particle size D50) of the YAG-based phosphor to be used for the light emitting device using the solid-state light emitting element as an excitation source of the phosphor is usually about 10 μm, and about 25 μm in large (for example, refer to PTLs 4 and 5). Note that a large number of particle shapes of this YAG-based phosphor are shapes derived from a crystal structure of the garnet. It is additionally described that an original shape derived from the crystal structure of the garnet is a polyhedron such as a rhombic dodecahedron and a biased polyhedron. However, the particle shape of the YAG-based phosphor is generally a shape of a pseudo-rhombic dodecahedron or a pseudo-biased polyhedron, in which edge portions are rounded. The particle shape is a spherical shape rather than a polyhedral shape.
In the light emitting device that uses blue light, which is radiated by the solid-state light emitting element, as the excitation source of the phosphor, a (Y,Gd)3Al2(AlO4):Ce3+ yellow phosphor in which a part of Y is replaced by Gd is frequently used. This is because white light with a relatively good color tone can be obtained by additive color mixture of the blue light radiated by the solid-state light emitting element and yellow light radiated by this yellow phosphor.
Moreover, a light emitting device has also been known, which is composed by combining a solid-state light emitting element and two or more garnet-based phosphors mutually different in luminescent color and composition and activated by Ce3+ for the purpose of controlling a color tone of output light of the light emitting device (for example, refer to PTL 1).
A load on the phosphor has tended to increase year by year as power of the light emitting device has been increasing. In recent years, total mineralization of the wavelength converter has been progressing (refer to PTLs 6 and 7).